The present invention relates to nuclear power reactors, and more particularly, to method and apparatus for sealing penetrations to the nuclear reactor vessel.
On pressurized water nuclear reactors, the control rod drive mechanisms are mounted on the reactor vessel head. The mechanisms are located in pressure boundary housings and provide controlling motion to the neutron absorbing control rods. The housing components are assembled to each other and joined with an omega type seal weld. These assemblies are then located and welded to the reactor vessel nozzles using omega seal welds. Maintenance on the control rod drive mechanisms, or replacement of the control rods or reactor vessel head, requires cutting of at least one omega weld. Due to cracking or other problems that have been found in penetrations to the reactor vessel head, some utilities have decided to replace the reactor vessel head. This approach requires the cutting of one of the omega welds on every one of dozens of control rod drive mechanism housings, so that these housings can be removed from the original head and re-installed on the new reactor vessel head. The re-welding of each omega seal, and the necessary inspection of the new weld, are laborious, time consuming processes if performed in the conventional manner, because they must be performed remotely in a confined, highly radioactive area.